The proxy secret wasn't generated properly, it has to be base64 encoded.
Therefore the following command is recommended:

```shell
cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 32 | head -n 1 | base64
```

References:
https://oauth2-proxy.github.io/oauth2-proxy/docs/configuration/overview#generating-a-cookie-secret